Nonlinear optical devices, e.g., frequency doublers, optical mixers and parametric oscillators, are of interest in both research and applied projects because of their ability to convert coherent optical radiation at one frequency into coherent optical radiation at another frequency. This ability is of interest because of the opportunities it affords both for expanding the number of wavelengths at which coherent radiation is available and for converting optical energy to a wavelength more convenient for device applications.
Many inorganic materials, such as quartz and lithium niobate, have nonlinear coefficients and exhibit optical nonlinearities. However, because the utility of these materials for device applications is generally proportional, at least at relatively low levels of incident radiation, to the magnitude of the nonlinear coefficients, materials are constantly being sought which exhibit nonlinear coefficients larger than the nonlinear coefficients of known materials and stable operation under diverse conditions, including high intensity incident radiation.
The possibility of using organic molecules in nonlinear optical devices has generated much interest recently because a large number of molecules are available for investigation. Some substituted aromatic molecules are known to exhibit large optical nonlinearities. The possibilty of such an aromatic molecule having large optical nonlinearities is enhanced if the molecule has donor and acceptor groups bonded at opposite ends of the conjugated system.
One such substituted aromatic molecule that is potentially interesting from an optical device point of view is para-nitroaniline. This molecule has a large molecular hyperpolarizability, and is transparent at many wavelengths of interest, including 0.532 .mu.m, which permits frequency doubling of the commonly used 1.064 .mu.m wavelength from a Nd:YAG laser. This molecule, however, crystallizes in centrosymmetric polymorphs. Because of this symmetry condition, the second harmonic coefficients are zero.
U.S. Pat. No. 4,199,698 discloses that the nonlinear optical properties of 2-methyl-4-nitroaniline (MNA) make it a highly useful material in nonlinear devices that convert coherent optical radiation including a first frequency into coherent optical radiation including a second frequency. The nonlinear devices have means for introducing coherent radiation in a first frequency into the MNA and means for utilizing coherent radiation emitted from the MNA at a second frequency.
U.S. Pat. No. 4,431,263 discloses that diacetylenes and polymers formed form diacetylenic species, which are amendable to close geometric, steric, structural, and electronic control, provide nonlinear optic, waveguide, piezoelectric, and pyroelectric materials and devices. Diacetylenes which are crystallizable into crystals having a noncentrosymmetric unit cell may form single crystals or may be elaborated into a thin film upon a substrate by the Langmuir-Blodgett technique. Such films may be polymerized either thermally or by irradiation for use in nonlinear optical and other systems. Diacetylenes are covalently bonded to substrates through the employment of silane species and subsequently polymerized to yield nonlinear optic and other devices having high structural integrity in addition to high efficiencies and optical effects.